feat(计费): 支持账号计费倍率快照与统计展示

- 新增 accounts.rate_multiplier（默认 1.0，允许 0）
- 使用 usage_logs.account_rate_multiplier 记录倍率快照，避免历史回算
- 统计/导出/管理端展示账号口径费用（total_cost * account_rate_multiplier）

backend/internal/service/gateway_service.go

@@ -2618,30 +2618,32 @@ func (s *GatewayService) RecordUsage(ctx context.Context, input *RecordUsageInpu
 	if result.ImageSize != "" {
 		imageSize = &result.ImageSize
 	}
+	accountRateMultiplier := account.BillingRateMultiplier()
 	usageLog := &UsageLog{
-		UserID:              user.ID,
-		APIKeyID:            apiKey.ID,
-		AccountID:           account.ID,
-		RequestID:           result.RequestID,
-		Model:               result.Model,
-		InputTokens:         result.Usage.InputTokens,
-		OutputTokens:        result.Usage.OutputTokens,
-		CacheCreationTokens: result.Usage.CacheCreationInputTokens,
-		CacheReadTokens:     result.Usage.CacheReadInputTokens,
-		InputCost:           cost.InputCost,
-		OutputCost:          cost.OutputCost,
-		CacheCreationCost:   cost.CacheCreationCost,
-		CacheReadCost:       cost.CacheReadCost,
-		TotalCost:           cost.TotalCost,
-		ActualCost:          cost.ActualCost,
-		RateMultiplier:      multiplier,
-		BillingType:         billingType,
-		Stream:              result.Stream,
-		DurationMs:          &durationMs,
-		FirstTokenMs:        result.FirstTokenMs,
-		ImageCount:          result.ImageCount,
-		ImageSize:           imageSize,
-		CreatedAt:           time.Now(),
+		UserID:                user.ID,
+		APIKeyID:              apiKey.ID,
+		AccountID:             account.ID,
+		RequestID:             result.RequestID,
+		Model:                 result.Model,
+		InputTokens:           result.Usage.InputTokens,
+		OutputTokens:          result.Usage.OutputTokens,
+		CacheCreationTokens:   result.Usage.CacheCreationInputTokens,
+		CacheReadTokens:       result.Usage.CacheReadInputTokens,
+		InputCost:             cost.InputCost,
+		OutputCost:            cost.OutputCost,
+		CacheCreationCost:     cost.CacheCreationCost,
+		CacheReadCost:         cost.CacheReadCost,
+		TotalCost:             cost.TotalCost,
+		ActualCost:            cost.ActualCost,
+		RateMultiplier:        multiplier,
+		AccountRateMultiplier: &accountRateMultiplier,
+		BillingType:           billingType,
+		Stream:                result.Stream,
+		DurationMs:            &durationMs,
+		FirstTokenMs:          result.FirstTokenMs,
+		ImageCount:            result.ImageCount,
+		ImageSize:             imageSize,
+		CreatedAt:             time.Now(),
 	}
 
 	// 添加 UserAgent